A motion transmission system in a bicycle comprises a chain extending between toothed wheels associated with the axle of the pedal cranks and with the hub of the rear wheel. When there is more than one toothed wheel at least one of axle of the pedal cranks and the hub of the rear wheel, and the motion transmission system is therefore provided with a gearshift, a front derailleur and/or a rear derailleur are provided for. In the case of an electronically servo-assisted gearshift, each derailleur comprises a chain guide element, also known as cage, movable to move the chain among the toothed wheels in order to change the gear ratio, and an electromechanical actuator to move the chain guide element. The actuator in turn typically comprises a motor, typically an electric motor, coupled with the chain guide element through a linkage such as an articulated parallelogram, a rack system or a worm screw system, as well as a sensor of the position, speed and/or acceleration of the rotor or of any moving part downstream of the rotor, down to the chain guide element itself. It is worthwhile noting that slightly different terminology from that used in this context is also in use.
Control electronics changes the gear ratio automatically, for example based on one or more detected variables, such as the travel speed, the cadence of rotation of the pedal cranks, the torque applied to the pedal cranks, the slope of the travel terrain, the heart rate of the cyclist and similar, and/or based on commands manually input by the cyclist through suitable control members, for example levers and/or buttons.
In order to drive the actuator, instead of assuming that the toothed wheels are equally axially spaced and therefore moving the chain guide element always by the same amount, the control electronics use a table of values containing, for each toothed wheel, the value that a variable of the derailleur must assume to position the chain in engagement with the toothed wheel. Such a value can be a differential value with respect to the adjacent toothed wheel, or it can be an absolute value with respect to a reference, for example with respect to a reference toothed wheel or to an end of stroke condition or to a condition of lack of excitation of the motor.
From the point of view of magnitude, the command value of the actuator can for example be the distance traveled by a mobile point taken as a reference on the derailleur, the number of steps or revolutions that the motor should be made to perform, a length of excitation time of the motor, the value of a supply voltage of a motor having an excursion proportional to the voltage, furthermore it can be the value emitted by the sensor associated with the motor, a numerical value stored in a register and representative of one of the aforementioned quantities, etc.
In particular, the motors of the actuators can be driven for a number of steps or for a length of excitation time or with a voltage that are appropriate for each upward or downward gearshifting and then automatically stopped, while the sensors are used to supply a feedback signal to the control electronics so that it can possibly take care of actuating the motors of the actuators again in case the intended position has not been reached, namely in case the aforementioned variable of the derailleur has not assumed the table value. This may for example be due to the fact that the resistant torque offered by the derailleur, which to a certain extent depends on how the cyclist is pedaling, was too high, greater than the maximum torque that can be delivered by the motors through the linkage.
The values of said table of values are nominal values, set in the factory, which take the number of toothed wheels in the derailleur (front or rear) and the respective thicknesses and pitches into account. Typically, such nominal values provide that, in the absence of the driving signal of the actuator, namely with command value at zero, the chain is in engagement with the toothed wheel having the smallest diameter, although as can be seen from the aforementioned examples, this condition is not necessary.
There are also some known gearshifts, for example from EP 1 426 284 A1 and from European patent application 11425204.2 not yet published at the priority date of the present application, wherein the nominal command values of the actuator are replaced in use by actual command values of the actuator, to take into account the variations in position of the toothed wheels, with respect to the nominal ones of a gearshift taken as a reference, due to various factors.
In the present description reference will be broadly made to “command values of the actuator”, meaning the actual ones where present, or the nominal ones, referring to a reference gearshift, in case the actual ones are not present.
More specifically, EP 1 426 284 A1 disclosed an electronically servo-assisted gearshift wherein a setting operating mode, an adjustment operating mode and a normal ride mode are implemented. In the setting mode, the chain is brought into alignment with a single preselected toothed wheel, preferably the one having the smallest diameter, and a biunique correspondence is set between the physical position of the actuator and the logic value associated with the gear ratio relative to the preselected toothed wheel, preferably zeroing a counter to the content of which the nominal values of the table are referred. In the adjustment mode, the chain is brought into engagement and alignment with a preselected toothed wheel and an adjustment variable (“offset”) of the logic value associated with the gear ratio relative to the preselected toothed wheel is set. During the normal ride mode, the actuator is moved into physical positions determined by the logic values associated with the toothed wheels as adjusted by the adjustment variables. In this way, the misalignments between the chain and one or more toothed wheels are compensated, caused for example by impacts or collisions or by small differences between the size and/or the position of a replaced toothed wheel and the replacement one.
Moreover, European patent application 11425204.2 discloses in particular a method for electronically controlling a bicycle gearshift and a gearshift that implements it, comprising the steps of:
a) detecting a first actual command value of an actuator such as to position a motion transmission chain in engagement with a first of at least three coaxial toothed wheels, and a second actual command value of the actuator such as to position the chain in engagement with a second of said toothed wheels,
b) for each toothed wheel, determining a nominal command value of the actuator theoretically such as to position the chain in engagement with said toothed wheel, and
c) computing an actual command value of said actuator at least for each of said toothed wheels other than the first and second toothed wheel, based on said nominal command values and said first and second actual command value.
According to such a document, in this way it is possible to take into account not only the size differences of the components of the frame and the mounting tolerances of the gearshift, but also size differences inside the pack of toothed wheels with respect to the theoretical reference gearshift on which the nominal values are based.
The Applicant has now perceived that, after the derailleur has reached the intended position at the end of its movement actuating a gearshifting—also when a feedback actuation of the derailleur is used and therefore the aforementioned movement includes movements correcting the resistant torque effect—, unintentional movements of the derailleur can follow, due to the elasticity of the linkage arranged between the motor of the actuator and the chain guide element, as well as due to vibrations due to the irregularity of the road surface. In the present description and in the attached claims, the derailleur is meant to be in intended position when the aforementioned variable of the derailleur has assumed the command value relative to the toothed wheel currently engaged.
Such unintentional movements, even when they can be considered micro-displacements, can cause imprecisions in the actuation of subsequent gearshifting, particularly when the command values are expressed as differential values between adjacent toothed wheels and subsequent undesired movements can add to one another. Furthermore, such movements from the intended position can cause an imprecise engagement if not even the disengagement of the chain from the toothed wheel, and/or a greater wear of the mechanical parts.
The technical problem at the basis of the invention is to counteract the undesired displacements of a derailleur of a bicycle gearshift.